1. Field of the Invention
The present invention relates to an optical disk apparatus for recording information in the form of very small pits being formed by irradiation of laser beams on a recording medium and different in optical properties, and for optically reproducing the recorded information.
2. Related Background Art
An optical disk apparatus is well known in which a narrow groove or a narrow protuberance (which is hereinafter referred to as a course) is formed on a recording medium of a disk type in the form of either a spiral or a concentric pattern, and in which very small pits, for example, of the width of 1 .mu.m and of the minimum length of 2 .mu.m, are formed along the course by a laser beam so as to record information on the basis of either the presence/absence or the length of the pit. In the optical disk apparatus of this type, the pits are formed by heat generated upon irradiation with the laser beam focused in a very small diameter on the recording medium. There have been proposed, for example, the following methods of thermal information recording, and some of them have been practically used.
(a) An indentation is formed by sloving or sublimating the medium with the heat to serve as the pit.
(b) An area is phase-transformed with the heat to differ in optical properties such as a refractive index and a reflectance from its original state, serving as the pit.
(c) An area is changed with respect to a direction of magnetization from its original state by degrading with the heat the coercive force of vertical magnetization directed up or down on the medium, reversing the direction of magnetization with application of reversed magnetic field, and then terminating the irradiation of the laser beam. The area serves as the pit. This method is called magneto-optical recording.
The recorded information may be reproduced by using the difference of the processed pit in optical properties from the other area, that is, by irradiating the medium with the laser beam and detecting an optical change of the beam reflected by or transmitted through the medium. In the specification, it should be noted that the term `pit` includes a simple `indentation`. Then, the difference in optical properties includes the case that diffraction or interference is caused with an indentation and not without an indentation.
The optical disk apparatus as described need a thermal recording device for recording and an optical reproducing device for reproduction accordingly. Many of such apparatus have a common device both for recording and for reproduction In the common device, it is usual that a beam head including a laser source and an optical system is commonly used both for recording and for reproduction. The beam head is used also for erasure in the conventional apparatus.
Two conditions as described below are known concerning an optimum recording emission power P.sub.W required upon the recording in such optical disk apparatus. The term `optimum recording emission power` is used to mean a recording emission power for forming a pit in the best condition.
(1) The faster a linear velocity v in the recording zone becomes, the higher the optimum recording emission power P.sub.W is. For example, there is the following relationship in the above-described magneto-optical recording. ##EQU1## A rotation angular velocity of the optical disk as the recording medium is constant in the conventional optical disk apparatus, which means that the velocity v becomes greater radially outwardly. Consequently, the more radially outward the recording position, the higher the optimum recording emission power P.sub.W required.
(2) The optimum recording emission power P.sub.W depends on a temperature of the recording medium. The higher the temperature is, the lower the optimum recording emission power P.sub.W is, and the lower the temperature, the higher the power.
From the above conditions (1) and (2), a required maximum value P.sub.Wmax of the power is the optimum recording emission power at the lowest operation temperature on the radially outermost course of the recording zone on the disk, and a required minimum value P.sub.Wmin is the optimum recording emission power at the highest operation temperation on the radially innermost course of the recording zone. Further, it is noted from the conditions (1) and (2) that the required emission power becomes higher either as the lowest operation temperature is set lower or as the recording zone of the disk is enlarged radially outward.
If the emission power is set too high upon the reproduction, the recorded information might be destroyed. The emission power upon the reproduction is thus set much lower than that upon the recording, also following the above-described conditions (1) and (2). The upper limit P.sub.R of the emission power for the reproduction can be obtained at each of radial positions of the recording zone at the maximum operation temperature. In order to cover both the entire temperature range and the entire recording zone required for the reproduction, the reproducing emission power must be lower than the upper limit power P.sub.RO in the radially innermost position. Generally, the reproducing emission power is limited below P.sub.RO by reducing an output from the laser source.
As described, the emission powers P.sub.Wmax, P.sub.Wmin and P.sub.RO are provided based on the operation temperature range and the size or recording zone of an optical disk medium. Normally, there is a great difference between P.sub.Wmax and P.sub.RO. The current technology can barely provide a semiconductor laser of a small maximum output. Therefore, it is necessary to increase beam emission efficiency from the semiconductor laser to an objective in order to supply the maximum emission power P.sub.Wmax of the beam head. However, the output from the laser source must be decreased to keep the reproduction emission power below P.sub.RO. It is known that signal to noise (S/N) ratio will be aggravated due to noise increase as the output of the semiconductor laser is excessively decreased. Therefore, since the output from the laser source must be kept over a certain level, the beam emission efficiency has to be lowered in accordance with the increase of the output. In case of the conventional optical disk apparatus having a single beam head, the operation temperature range or the recording zone is extremely restricted because of the limited adjustable range of the emission power of the beam head.
There is a technique known for preventing the decrease of S/N ratio due to noise in the low laser power, e.g, superposing a high frequency on the laser. However, such a technique requires a separate, complex circuit.